Among III-V compound semiconductors, GaN is known to have a larger energy band gap of 3.4 eV, and an indirect transition conductive band level thereof is at 1.5 eV or more over the valence band level. Besides, a saturation electron velocity of GaN is about 2.5×107 cm/sec, and a breakdown voltage thereof is about 5×106 V/cm, both greater than those of Si, GaAs and SiC.
Accordingly, GaN can be used as a material for high-speed power transistors.
As one electronic device using GaN, there are a MESFETs (Metal-Semiconductor Field Effect Transistor) as disclosed in Patent document 1 and a high electron mobility transistor as disclosed in Patent document 2.
The MESFETs has, for example, as shown in FIG. 19, a un-doped GaN layer 102 and an n type GaN channel layer 103 formed sequentially on a substrate 101, a gate electrode 104 in Schottky contact with the n type GaN channel layer 103, and a source electrode 105 and a drain electrode 106 which are formed to the respective sides of the gate electrode 104 and are in ohmic contact with the channel layer 103. The reference numeral 107 in FIG. 19 denotes a depletion layer.
In addition, the high electron mobility transistor has an un-doped GaN electron moving layer 112 and an n type AlGaN electron supplied layer 113 sequentially deposited on a substrate 111 as shown in FIG. 20, and further has a gate electrode 114 in Schottky contact with the n AlGaN electron supplied layer 113 and a source electrode 115 and a drain electrode 116 which are formed to the respective sides of the gate electrode 114 and are in ohmic contact with the n AlGaN electron supplied layer 113. In FIG. 20, the reference numeral 117 denotes a two-dimensional electron gas and the reference numeral 118 denotes a depletion layer.
The transistors shown in FIGS. 19 and 20 are both general normally-on transistors. When a transistor is used as a power supply device for general purpose, a normally-off type is required in terms of fail safe. The normally-off field effect transistor is disclosed in Patent documents 3 and 4 in which an insulating film is placed between a gate electrode and a GaN layer.
However, the conventional normally-off transistors using GaN materials have a larger parasitic resistance, and there is a problem that adequate off characteristics cannot be assured when the gate voltage is 0 V.
Meanwhile, Non-patent document 1 discloses a normally-off high electron mobility transistor in which a gate threshold voltage becomes 0 V by performing fluorine plasma processing after forming a gate electrode.
Used as a field effect transistor using a III-nitride semiconductor are an AlGaN/GaN HEMT (for example, refer to Non-patent document 2) and a GaN MOSFET (refer to Non-patent document 3). These have higher saturation electron mobility and higher dielectric breakdown voltage than those of conventionally using a III compound semiconductor such as Si, GaAs, InP and the like, and they are suitable for power devices.    Patent Document 1: Japanese Patent Publication No. 9-307097    Patent Document 2: Japanese Patent Publication No. 2003-59948    Patent Document 3: Japanese Patent Publication No. 2001-320054    Patent Document 4: Japanese Patent Publication No. 2004-260140    Non-Patent Document 1: Proceedings of The 53th Meeting of Applied Physics and Related Societies, (2006, Spring, Musashi institute of technology, third edition, 14a-ZE-17, p. 1513)    Non-Patent Document 2: M. Kuraguchi et al., “Normally-off GaN-MISFET with well-controlled threshold voltage”, International Workshop on Nitride Semiconductors 2006 (IWN 2006), Oct. 22-27, 2006, Kyoto, Japan, WeED1-4    Non-Patent Document 3: Huang W, Khan T, Chow T P: Enhancement-Mode n-Channel GaN MOFETs on p and n-GaN/Sapphire substrates.    In: 18th International Symposium on Power Semiconductor Devices and ICs (ISPSD) 2006 (Italy), 10-1.